When carrying out a machining operation with a laser beam machining device, a laser beam is focused at a point on a workpiece by a focusing lens or the like, whereby the temperature of the irradiated portion is raised, and accordingly, machining operations such as drilling and cutting, etc. can be effected by evaporating a portion of a workpiece by a laser beam converged onto a very small spot, and welding can be effected by slightly shifting the focal point to thereby maintain the fused state of the workpiece. Therefore, various machining operations can be effected regardless of hardness of the material of the workpieces.
Metals such as copper and aluminum, etc., however, have a low capacity for the absorption of a laser beam at normal temperatures, and thus more than 80% of the beam is reflected immediately after the irradiation of a laser beam. Nevertheless, once the metals are heated, the absorption capacity thereof is increased, and thus a required machining operation can be effected. A specific example of this phenomenon is shown in FIGS. 4(a) and 4(b).
FIG. 4(a) is a graph showing a change of the capacity for the absorption by a workpiece of a laser beam, wherein a CO.sub.2 gas laser beam is used as the laser beam and aluminum is used as the workpiece. In the figure, 40 denotes the characteristic of an incident laser beam irradiated to the workpiece, i.e., a pulse waveform having a width of a period of from a time t0 to a time t2 and a peak value of Pi, and 41 denotes the characteristic of a reflected laser beam. The level of the reflected laser beam rises, together with the incident laser beam, from the time t0 up to a level Pr, but as the absorption capacity of the workpiece increases thereafter, abruptly drops when close to the time t1.
A change of the reflectivity in the above case is shown in FIG. 4(b). In the figure, the times t0 to t2 correspond respectively to those designated by the same symbols in FIG. 4(a). As illustrated, the reflectivity is close to 1 at the initial stage of the laser beam irradiation, drops approximately to zero at the time t1, and again approaches 1 after the irradiation is ended.
The above-mentioned reflected laser beam is allowed to pass through the focusing lens and bender mirrors, etc., and is fed back to an oscillator through an output mirror, and therefore, if the peak level Pr of the reflected laser beam, shown in FIG. 4(a), is high, the optical components can be damaged by an abnormal increase of the laser power in the oscillator.
Furthermore, while the power level of the reflected laser beam is high, i.e. while the reflectivity is high, a required machining operation cannot be satisfactorily effected.